1. Field of the Invention
The present invention relates to an optical receiver for receiving an optical signal in the optical communication system.
2. Related Art
Japanese Patent published as JP-2003-198279A has disclosed one type of optical receiver with a circuit for monitoring an optical input level, in which the output of the differential amplifier of the voltage follower is positively shifted to reduce an error due to the non-linearity of the output thereof by inserting a diode between the output of the differential amplifier of the voltage follower and the output of the feedback loop.
Conventional monitoring unit such as those disclosed in the Japanese Patent mentioned above includes a voltage follower configured by a differential amplifier whose non-inverting input is connected to a resister that converts a photocurrent generated by a photodiode by receiving the optical input signal into a voltage signal with a reference to the ground. When the optical input level is quite small for the photodiode to generate only a faint photocurrent, a voltage drop generated in the resistor by this photocurrent becomes quite small and the input level of the voltage follower is very close to the ground, to cause an output voltage error because of the nonlinear characteristic of the differential amplifier when its output is very small.
In the optical receiver disclosed in the Japanese Patent mentioned above, by inserting a diode in the output of the differential amplifier, the output thereof may be within a range securing the linearity even when the output of the feedback loop closes the ground level because the output of the differential amplifier is positively shifted by a forward voltage of the diode. However, the input of the differential amplifier is left in a very small voltage close to an input offset voltage of the differential amplifier when the optical input level is small. Thus, it is quite hard to monitor the optical input level in accurate when the level thereof is quite small.
Accordingly, the present invention, based on subjects mentioned above, is to provide an optical receiver with a function to accurately monitor the optical signal with a quite wide dynamic range.